# properties of electromagnetic waves

## speed of electromagnetic waves

All electromagnetic waves travel at the same speed through empty space. That speed, called the speed of light, is 300 million meters per second (3.0 108 m/s). Nothing else in the universe is known to travel this fast. If you could move that fast, you would be able to travel around Earth 7.5 times in just 1 second! The sun is about 150 million kilometers (93 million miles) from Earth, but it takes electromagnetic radiation only 8 minutes to reach Earth from the sun. Electromagnetic waves travel more slowly through a medium, and their speed may vary from one medium to another. For example, light travels more slowly through water than it does through air (see Figure 21.4). You can learn more about the speed of light at this URL: http://videos.howstuffworks.com/discovery/29407-assignme

## wavelength and frequency of electromagnetic waves

Although all electromagnetic waves travel at the same speed, they may differ in their wavelength and frequency.

## defining wavelength and frequency

Wavelength and frequency are defined in the same way for electromagnetic waves as they are for mechanical waves. Both properties are illustrated in Figure 21.5. Wavelength is the distance between corresponding points of adjacent waves. Wavelengths of electromagnetic waves range from many kilometers to a tiny fraction of a millimeter. Frequency is the number of waves that pass a fixed point in a given amount of time. Frequencies of electro- magnetic waves range from thousands to trillions of waves per second. Higher frequency waves have greater energy.

## speed wavelength and frequency

The speed of a wave is a product of its wavelength and frequency. Because all electromagnetic waves travel at the same speed through space, a wave with a shorter wavelength must have a higher frequency, and vice versa. This relationship is represented by the equation: Speed = Wavelength Frequency The equation for wave speed can be rewritten as: Frequency = Speed Speed or Wavelength = Wavelength Frequency Therefore, if either wavelength or frequency is known, the missing value can be calculated. Consider an electromag- netic wave that has a wavelength of 3 meters. Its speed, like the speed of all electromagnetic waves, is 3.0 108 meters per second. Its frequency can be found by substituting these values into the frequency equation: Frequency = 3.0 108 m/s = 1.0 108 waves/s, or 1.0 108 hertz (Hz) 3.0 m You Try It! Problem: What is the wavelength of an electromagnetic wave that has a frequency of 3.0 108 hertz? For more practice calculating the frequency and wavelength of electromagnetic waves, go to these URLs:

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

How long does it take electromagnetic radiation to reach Earth from the sun?

``````a. 1 second

b. 7.5 seconds

-->  c. 8 minutes

d. 93 minutes
``````

What happens to light when it passes from water to air?

``````a. Its speed decreases.

b. Its frequency increases.

c. Its wavelength decreases.

-->  d. none of the above
``````

If the frequency of an electromagnetic wave is 6.0 108 hertz, what is its wavelength?

``````-->  a. 0.5 m

b. 1.0 m

c. 1.5 m

d. 2.0 m
``````

Electromagnetic waves with the lowest frequencies may have wavelengths as long as

``````-->  a. many kilometers.

b. a few meters.

c. a couple of centimeters.

d. a fraction of a millimeter.
``````

Which statement about electromagnetic waves is true?

``````a. An electromagnetic wave with a shorter wavelength has a lower frequency.

-->  b. All electromagnetic waves travel at the same speed across space.

c. All electromagnetic waves are harmful.

d. none of the above
``````

Some electromagnetic waves are extremely harmful.

``````-->  a. true

b. false
``````

All electromagnetic waves travel at the same speed across space.

``````-->  a. true

b. false
``````

It takes electromagnetic radiation 93 minutes to reach Earth from the sun.

``````a. true

-->  b. false
``````

All electromagnetic waves have the same wavelength.

``````a. true

-->  b. false
``````

Electromagnetic waves travel more quickly through a medium than they do across space.

``````a. true

-->  b. false
``````

The frequencies of electromagnetic waves range from 1 to 100 hertz.

``````a. true

-->  b. false
``````

The highest-frequency electromagnetic waves may have frequencies of trillions of hertz.

``````-->  a. true

b. false
``````

The frequency of an electromagnetic wave is inversely related to its wavelength.

``````-->  a. true

b. false
``````

Light is diffracted when it passes from air to water at an angle.

``````a. true

-->  b. false
``````

Light has a faster speed across space than do any other wavelengths of electromagnetic radiation.

``````a. true

-->  b. false
``````

Electromagnetic waves travel at the same speed in all media.

``````a. true

-->  b. false
``````

If you know only the wavelength of an electromagnetic wave, you can calculate its frequency.

``````-->  a. true

b. false
``````

distance between corresponding points of adjacent waves

``````a. speed of light

-->  b. wavelength

c. wave frequency

d. wave speed

e. light

f. medium
``````

fastest known speed in the universe

``````-->  a. speed of light

b. wavelength

c. wave frequency

d. wave speed

e. light

f. medium
``````

matter through which an electromagnetic wave may travel

``````a. speed of light

b. wavelength

c. wave frequency

d. wave speed

e. light

-->  f. medium
``````

number of waves that pass a fixed point in a given amount of time

``````a. speed of light

b. wavelength

-->  c. wave frequency

d. wave speed

e. light

f. medium
``````

``````a. speed of light

b. wavelength

c. wave frequency

d. wave speed

-->  e. light

f. medium
``````

value that equals wavelength multiplied by wave frequency

``````a. speed of light

b. wavelength

c. wave frequency

-->  d. wave speed

e. light

f. medium
``````

Properties of electromagnetic waves include

``````a. speed.

b. wavelength.

c. frequency.

-->  d. all of the above
``````

Light slows down when it

``````a. travels across space.

-->  b. passes from air to water.

c. passes from water to air.

d. two of the above
``````

Electromagnetic waves may vary in their

``````a. speed across space.

b. energy level.

c. frequency.

-->  d. two of the above
``````

Wavelengths of electromagnetic waves range from

``````-->  a. many kilometers to a tiny fraction of a millimeter.

b. millions of kilometers to several meters.

c. several meters to a few millimeters.

d. one kilometer to one millimeter.
``````

The highest-frequency electromagnetic waves have a frequency of

``````a. hundreds of waves per second.

b. thousands of waves per second.

c. millions of waves per second.

-->  d. trillions of waves per secon
``````

If the wavelength of an electromagnetic wave is 3.0 m, what is its frequency?

``````a. 9.0  102 hertz

b. 6.0  104 hertz

c. 3.0  106 hertz

-->  d. 1.0  108 hertz
``````

If the frequency of an electromagnetic wave is 3.0 108 hertz, what is its wavelength?

``````a. 1 mm

b. 1 cm

-->  c. 1m

d. 1 km
``````

## diagram questions

No diagram questions associated with this lesson